Midterm results of the Synergy cementless tapered stem: stress shielding and bone quality.

Background: Tapered femoral stems have been shown to produce less thigh pain and stress shielding than other cement-less stem designs. The purpose of this study was to examine the performance of this type of femoral stem in Japanese patients.

Methods: A series of 40 total hip arthroplasties with a Synergy tapered femoral component were performed in 33 patients between March 1999 and February 2001 at our institution. Three hips (in three patients) were lost to follow-up, so 37 hips in 30 patients were followed for an average of 70 months. The patients' average age at the time of surgery was 59 years (range 43-80 years). Clinical evaluation included the Japanese Orthopaedic Association (JOA) Hip Score and the incidence of thigh pain. Preoperative radiographic examination included the cortical index and Dorr's bone type. Postoperative radiographic examination included evaluation of biological fixation, spot welds, cortical hypertrophy, and stress shielding.

Results: The average JOA Hip Score improved significantly from 35 preoperatively to 91 postoperatively. No patient suffered thigh pain. All femoral components were classified as bony stable. Spot welds and cortical hypertrophy were commonly found in the middle to the distal portion of the component. Severe (third and fourth degree) stress shielding was observed in 24.3% of the cases. A low cortical index, a Dorr type C femur, and a large stem size were associated with severe stress shielding.

Conclusions: The midterm clinical results with the Synergy femoral component were satisfactory for Japanese patients. Although severe stress shielding was frequently observed in patients with poor bone quality, stem stability was not affected. Extent of grit blasting and stem length may be factors causing severe stress shielding in Japanese patients with poor bone quality. In conclusion, poor bone quality is a less favorable indication for the cementless tapered stem.